Lithography methods are widely used in the production of microscopic structures in a variety of electronic devices such as semiconductor devices and liquid crystal devices, and ongoing miniaturization of the structures of these devices has lead to demands for her miniaturization of the resist patterns used in these lithography processes. With current lithography methods, using the most up-to-date ArF excimer lasers, fine resist patterns with a line width of approximately 90 nm are able to be formed, but in the future, even finer pattern formation will be required.
In order to enable the formation of these types of ultra fine patterns, the development of appropriate exposure apparatus and corresponding resists is the first requirement.
In the case of resists, chemically amplified resists, which includes an acid generator which generates acid on exposure to radiation and a base resin which exhibits changed alkali solubility under the action of acid, are attracting considerable attention, and development of these resists is flourishing. According to the chemically amplified resists, high levels of resolution are achieved, a catalytic reaction or chain reaction of an acid generated by irradiation can be utilized, a quantum yield is 1 or greater, and high sensitivity can be achieved.
In positive chemically amplified resists, resins having acid-dissociable, dissolution-inhibiting groups are the most commonly used. Examples of known acid-dissociable, dissolution-inhibiting groups include acetal groups such as ethoxyethyl groups, tertiary alkyl groups such as tert-butyl groups, as well as tert-butoxycarbonyl groups and tert-butoxycarbonylmethyl groups. Furthermore, structural units derived from tertiary ester compounds of (meth)acrylic acid, such as 2-alkyl-2-adamantyl (meth)acrylates, are widely used as the structural units containing an acid-dissociable, dissolution-inhibiting group within the resin component of conventional ArF resist compositions, as disclosed in the patent reference 1 listed below.
Herein, the term “(meth)acrylic acid” is a generic term that includes both the acrylic acid in which a hydrogen atom is bonded to the α-position, and the methacrylic acid in which a methyl group is bonded to the α-position. Also, the term “(meth)acrylate ester” is a generic term that includes both the acrylate ester in which a hydrogen atom is bonded to the α-position, and the methacrylate ester in which a methyl group is bonded to the α-position.
On the other hand, in the case of the exposure apparatus, techniques such as shortening the wavelength of the light source used, and increasing the diameter of the lens aperture (NA) (namely, increasing NA) are common. For example, for a resist resolution of approximately 0.5 μm, a mercury lamp for which the main spectrum is the 436 nm g-line is used, for a resolution of approximately 0.5 to 0.30 μm, a similar mercury lamp for which the main spectrum is the 365 nm i-line is used, for a resolution of approximately 0.30 to 0.15 nm, 248 nm KrF excimer laser light is used, and for resolutions of approximately 0.15 μm or less, 193 nm ArF excimer laser light is used. In order to achieve even greater miniaturization the use of F2 excimer laser light (157 ma), Ar2 excimer laser light (126 nm), EUV (extreme ultraviolet radiation: 13.5 nm), EB (electron beams), and X-rays and the like is also being investigated.
However, shortening the wavelength of the light source requires a new and expensive exposure apparatus. Furthermore, if the NA value is increased, then because the resolution and the depth of focus range exist in a trade-off type relationship, even if the resolution is increased, a problem arises in that the depth of focus reduces.
Against this background, a method known as immersion exposure has been reported (for example, see non-patent references 1 to 3). This method includes a step in which exposure (immersion exposure) is conducted with the region between the lens and the resist film disposed on top of the wafer, which has conventionally been filled with air or an inert gas such as nitrogen, filled with a solvent (an immersion medium) that has a larger refractive index than the refractive index of air.
According to this type of immersion exposure, it is claimed that higher resolutions equivalent to those obtained using a shorter wavelength light source or a larger NA lens can be obtained using the same exposure light source wavelength, with no reduction in the depth of focus. Furthermore, immersion exposure can be conducted using existing exposure apparatus. As a result, it is predicted that immersion exposure will enable the formation of resist patterns of higher resolution and superior depth of focus at lower costs. Also, in the production of semiconductor elements, which requires enormous capital investment, immersion exposure is attracting considerable attention as a method that offers significant potential to the semiconductor industry, both in terms of cost and in terms of lithography properties such as resolution. Currently, water is mainly used as the immersion medium for immersion lithography.
[Patent Reference 1]
Japanese Unexamined Patent Application, First Publication No. Hei 10-161313
[Non-Patent Reference 1]
Journal of Vacuum Science & Technology B (U.S.), 1999, vol. 17, issue 6, pp. 3306 to 3309.
[Non-Patent Reference 2]
Journal of Vacuum Science & Technology B (U.S.), 2001, vol. 19, issue 6, pp. 2353 to 2356.
[Non-Patent Reference 3]
Proceedings of SPIE (U.S.), 2002, vol. 4691, pp. 459 to 465.